fff-wireless: Provide device-specific WiFi settings and set TX power

Like the network.* files, this introduces wifi.* files to set
device-specific settings.

In contrast to the former, the wifi.* settings are only run during
firstboot and only if there is a file present.

In this patch, we set the antenna gain values for the devices not
treated correctly in OpenWRT.

Side conditions:
- The script requires 50-fff-boardname and 60-fff-wireless
  beforehand, so I chose 62 as number.
- The wifi command is not necessary, since after firstboot
  configurehood will run "wifi" in any case

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Fabian Bläse <fabian@blaese.de>
Signed-off-by: Tim Niemeyer <tim@tn-x.org>
- Changed to base on the ubnt board names
Reviewed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>